1. Field of the Invention
The present invention relates to a sensing arrangement for sensing a write enable ring in a magnetic tape reel structure in which a sensing ring is provided at a tape roll within a magnetic tape reel, the sensing ring being displaced in the axial direction given an inserted write enable ring and the outside of the sensing ring exhibiting a surface having high reflectivity, and wherein the surface is sensed with the use of an opto-electronic sensing element.
2. Description of the Prior Art
It is generally well known in the art to protect data stored on a magnetic tape against inadvertent overwriting or erasure in that a write enable ring is concentrically inserted into a recess on the rear of an appertaining magnetic tape reel. Data can be recorded on the magnetic tape or erased only given an inserted write enable ring. The magnetic tape can only be read without the write enable ring.
It is conceivable to opto-electronically check the presence and absence of the write enable ring with the use of a sensing ring and an appertaining opto-electronic sensing element. The sensing ring exhibits a diameter that corresponds to that of the write enable ring and is disposed at the roll of tape axially displaceable at the rear of the magnetic tape reel. When a write enable ring is not inserted, then the sensing ring is pushed into the corresponding recess for the write enable ring at the rear of the magnetic tape reel as a result of spring power of one or more springs. An insertion of the sensing ring into the recess is prevented, however, when the write enable ring is inserted. The displacement of the sensing ring in the axial direction is identified by an opto-electronic sensing element. To this end, the exterior, for example, of the sensing ring is provided with two surfaces in the axial direction which exhibit differing reflectivity. For example, the front half of the exterior is provided with a black color, whereas the rear half is mirrored. The opto-electronic sensing element is preferably designed as a reflecting light-barrier that senses the mirrored surface as a function of the axial displacement of the sensing ring, given lack of a write enable ring and senses the black surface given an inserted write enable ring and, therefore, generates a corresponding scanning signal that indicates the presence or absence of the write enable ring.
Only a static check of the presence or absence of the write enable ring, however, can be carried out with the assistance of such a sensing arrangement. It exhibits a relatively low operating reliability since the outage of the opto-electronic sensing elements cannot be perceived without the provision of additional steps. In this case, a wrong signaling can occur so that there is a risk that protected data on the magnetic tape will be inadvertently overwritten or erased.
A German patent application No. P 32 16 280.4, on which the present invention is based and which is fully incorporated herein by this reference, discloses a sensing device wherein the sensing ring exhibits mutually offset surfaces having different reflectivities that are scanned with the assistance of two opto-electronic sensing elements. As a function of the displacement of the sensing ring, respective surfaces having different or identical reflectivity are sensed and, given a rotation of the sensing ring, respective surfaces having different reflectivity are sensed in chronological succession so that the presence and absence of the write enable ring can be both statically and dynamically sensed.